This Invention relates to the adjustment of filters, especially in ways that can address manufacturing variations. The filters may be usable in transceivers for transmitting and/or receiving radio signals.
FIG. 1 shows schematically the structure of one example of a radio transceiver. The transceiver has an antenna 1 and a signal processing unit 2 for baseband processing of received signals and signals that are to be transmitted. Between the antenna and the signal processing unit are a receive chain 3 and a transmit chain 4, which are connected to the antenna 1 by a duplexer 5. The receive chain 3 converts received radio frequency (RF) signals down to baseband for further processing by the signal processing unit 2. The transmit chain 4 converts signals from baseband to AS for transmission from the antenna 1. The receive chain comprises an input emplifier 7 which amplifies the received signal, a mixer 8 which mixes the amplified signals with a signal from oscillator 9 to convert to Intermediate frequency (IF), and a mixer 10 which mixes the IF signal with a signal from oscillator 11 to convert to baseband. Between each of the units 7, 8, 10 and 2 is a bandpass filter 12, 13, 14 to remove off-band interference and image frequencies. The transmit chain comprises a mixer 17 which mixes the baseband signals with a signal from oscillator 18 to upconvert to RF and an output amplifier 19 which amplifies the RF signals for transmission. Filters 20, 21, 22 can also be included in the transmit chain.
In many applications it would be desirable, in order to reduce size and cost, to implement the entire transceiver, or at least the receive and transmit chains on a single integrated circuit (IC) One particular difficulty in designing such an IC is the implementation of the filters and the oscillators, and especially of circuitry intended for setting the operational frequencies of those components. In many implementations of transceiver circuits the components of the filters (especially the RF filters 12, 20, 21) are implemented by passive components rather than as active filters. Often these are provided as discrete off-chip components such as capacitors, inductors and ceramic or SAW (surface acoustic wave) filters even if the remainder of the transmit and receive chains is implemented on a single integrated circuit.
The approach of Implementing fitters by means of discrete components generally provides filters that have superior response, noise or linearity characteristics. However, there is a need to adjust such filters after manufacturing in order to tune them to the desired frequency response. For this reason adjustable discrete components such as trimmer capacitors are generally also provided to allow the filters to be tuned to the desired frequency response. This approach has a number of disadvantages. First, during manufacturing the fitting of the discrete components requires additional processing steps. Second, the discrete components demand additional space in the radio device, increasing Its overall size. Third, additional time and additional processing stations are needed to adjust the filters mechanically to the desired response during the manufacturing process,
In several situations there is correspondence between adjustment desired to be made to an oscillator of the transceiver (e.g. oscillator 18) and a filter of the transceiver (e.g. filter 20) This can arise because (i) when a channel is selected for transmission and reception the filter and the oscillator may have to be adjusted correspondingly so that the pass band of the filter and the oscillation frequency of the oscillator are the same or related by simple algebraic expressions and (ii) because when the filter and the oscillator are on the same integrated circuit they are likely to be subject to similar systematic errors due to environmental factors.
It would be desirable to employ a filter that has the generally superior characteristics of a filter formed from discrete components but that can be implemented on chip, an d that could be adjusted efficiently
According to the present invention there is provided a radio transmitter and/or receiver comprising: an oscillator tuning circuit comprising an adjustable capacitor whose capacitance is adjustable by means of a first tuning signal; a filter tuning circuit comprising an adjustable capacitor whose capacitance is adjustable by means of a second tuning signal; an oscillator whose operational frequency is dependent on the reactance of th e oscillator tuning circuit; a filter for filtering signals in the course of transmission and/or reception, and whose response is dependant on the reactance of the filter tuning circuit; and a tuning unit for generating the first and second tuning signals, wherein at least a part of the filter tuning circuit is a replica of at least a part of the oscillator tuning circuit and the tuning circuit is capable of generating one of the first and second tuning signals in dependence on the other of the tuning signals.
The said part of the oscillator tuning circuit suitably comprises a plurality of selectively engageable reactive elements on which the reactance of the filter tuning circuit is dependent. The reactive elements may be capacitors or inductors. The reactive elements are preferably discrete components. The reactive elements are preferably formed on a single integrated circuit.
The said part of the filter tuning circuit suitably comprises a plurality of selectively engageable reactive elements on which the reactance of the filter tuning circuit is dependent. The reactive elements may be capacitors or inductors The reactive elements are preferably discrete components. The reactive elements are preferably formed on a single integrated circuit. The said part of the filter tuning circuit preferably comprises a plurality of reactive elements each corresponding to a reactive element of the oscillator tuning circuit.
Most preferably the reactive elements of the said part of the filter tuning circuit are nominally of a common scale with respect to the corresponding elements of the said part of the oscillator tuning circuit The reactive elements of the said part of the filter tuning circuit may be nominally identical to the corresponding elements of the said part of the oscillator tuning circuit. The said reactive elements of the said part of the filter tuning circuit are nominally scaled as an integer multiple of the corresponding elements of the said part of the oscillator tuning circuit. The said reactive elements of the said part of the oscillator tuning circuit may be nominally scaled as an integer multiple of the corresponding components of the said part of the filter tuning circuit. The scaling may be in size, nominal value or actual value.
Each of the said reactive elements of the oscillator tuning circuit may be selectively engageable by a respective oscillator tuning switch means (e.g a transistor or other preferably electrically actuable switch) connected in series with it Preferably each of the said oscillator tuning switch means is responsive to a digital component of the second tuning signal. Preferably each of the said reactive elements of the filter tuning circuit is selectively engageable by a respective filter tuning switch means connected in series with it. Each of the said filter tuning switch means is suitably responsive to a digital component of the first tuning signal. The first and second tuning signals may be provided on a plurality of individual digital lines. The first and second tuning signals may each comprise a plurality of digital signal components.
Preferably the operational frequency of the oscillator is an integer multiple or fraction of the frequency that lies in or near the pass band of the filter.
Most preferably the filter tuning circuit and the oscillator tuning circuit are formed on a single Integrated circuit.
The filter is suitably part of a radio receiver unit of the transceiver, which is preferably a zero or near zero intermediate frequency receiver unit. The filter may be a passive filter. The oscillator may be a local oscillator for the transmitter and/or receiver.